1. Field of the Invention
The present invention relates to a brazing apparatus for metal workpieces such as aluminum, copper, copper alloys, iron or stainless steel and so forth in which a convection type furnace (thermal convection furnace) is employed, more specifically to a brazing apparatus for brazing aluminum parts on large-sized aluminum workpieces such as a heat exchanger for an automobile, utilizing an index type (intermittent motion type) convection furnace.
2. Description of the Related Art
A convection furnace for brazing large-sized metal workpieces made of aluminum such as a heat exchanger for an automobile etc. is generally designed to heat the workpieces fixed or carried within a brazing chamber, by circulating using a fan etc. inside the brazing chamber a heating medium gas composed of an inactive gas for example nitrogen gas etc. heated in a combustion chamber.
Brazing temperature varies depending on metal materials, brazing materials and types of flux, and normally it is 550xc2x0 C. to 640xc2x0 C. for brazing aluminum of AA1000 series with a fluoride flux, and 700xc2x0 C. to 850xc2x0 C. for copper and stainless steel.
For example, brazing temperature for aluminum of AA1100 with aluminum of AA4045 or 4047 as brazing material and a fluoride as flux is approx. 600xc2x0 C. In a convection type brazing furnace, workpieces are usually heated up to approx. 350xc2x0 C. in a preheating furnace. The workpieces carried into a brazing chamber of the brazing furnace is rapidly heated by heating medium gas heated to approx. 610xc2x0 C. to 620xc2x0 C., so that the workpieces temperature is raised up to 600xc2x0 C. in several minutes.
Normally such temperature is maintained during the heating to fuse the flux and to complete the brazing, however despite maintaining the furnace temperature at 600xc2x0 C. by controlling the heating medium gas temperature, a temperature slope of 30xc2x0 C. to 40xc2x0 C. is generated among the surface of the workpieces which the heating medium gas directly hits, the opposite side thereof and interior sections of the workpieces, because of which a flow of the brazing material becomes uneven, and resultantly brazing quality is lowered.
The inventors of the present invention proposed method and an apparatus by which workpieces temperature can be made uniform and a high-quality brazing can be executed, wherein temperature of heating medium gas to be applied to the workpieces in a convection type brazing furnace is varied up and downward in small increments (hereinafter referred to as xe2x80x9cpulse heatingxe2x80x9d) during a temperature raising process and after reaching a predetermined brazing temperature. (Ref. JP-A No.2001-340958: U.S. Pat. No. 2001-0051323)
However, according to the convection type brazing furnace disclosed in JP-A No.2001-340958 wherein heating medium gas temperature is varied up and downward to perform the pulse heating, lowering the heating medium gas temperature does not instantly lower the brazing chamber temperature because of a large thermal capacity of the brazing chamber itself, therefore it takes some time before reaching a desired temperature. Likewise, when the heating medium gas temperature is raised also, there is a certain time lag before the brazing chamber temperature reaches a desired level.
Because of the inadequate thermal response, by the conventional method of varying the heating medium gas temperature up and downward it is difficult to shorten a cycle time (cycle of high-temperature heating and low-temperature heating) of the pulse heating, therefore a brazing time inevitably becomes longer, resulting in a lower productivity.
In view of the foregoing, it is an object of the invention to provide a convection type brazing apparatus for metal workpieces that can shorten an operation time for brazing metal workpieces such as aluminum, copper, copper alloys, iron or stainless steel and so forth to achieve a higher productivity, and enables uniform heating of the workpieces to improve a yield of products.
As a result of persistent studies for achieving the mentioned object, the inventors of the present invention have discovered that in a brazing operation for metal workpieces such as aluminum, copper, copper alloys, iron or stainless steel, etc. utilizing a convection furnace, intermittently supplying heating medium gas can effectively shorten a cycle time of the pulse heating and minimize a temperature slope of the workpieces especially large-sized workpieces, and further shorten the brazing operation time to improve productivity.
Accordingly, the invention provides a convection type brazing apparatus for metal workpieces having a brazing furnace provided with at least a thermal medium gas heating chamber, a brazing chamber and a heating medium gas circulation path through which the heating medium gas heated in the thermal medium gas heating chamber returns to the thermal medium gas heating chamber via the brazing chamber, comprising a heating medium gas circulation path switching mechanism for periodically switching the heating medium gas circulation path so that a blowing phase during which the heating medium gas heated in the thermal medium gas heating chamber is blown into the brazing chamber and a suspension phase during which blow of the heating medium gas into the brazing chamber is suspended are alternately repeated; and a bypass circulation path through which the heating medium gas returns directly to the thermal medium gas heating chamber halfway of the heating medium gas circulation path without running through the brazing chamber during the suspension phase.
For the heating medium gas circulation path switching mechanism, an ordinary switching valve such as a damper valve, slide valve, butterfly valve, etc. may be employed. Also, though the heating medium gas circulation path switching mechanism can have separate valves in the heating medium gas circulation path and the bypass circulation path, it is more economical to provide a single valve to be used in common for switching the both circulation paths.
Also, the heating medium gas circulation path switching mechanism comprises means for controlling for closing the bypass circulation path while the heating medium gas circulation path is opened during the blowing phase and opening the bypass circulation path while the heating medium gas circulation path is closed during the suspension phase.
The heating medium gas circulation path switching mechanism controlling means is provided with a function to set a cycle time of the blowing phase and the suspension phase of the heating medium gas circulation path in a range of 5 seconds to 5 minutes.
In an ordinary convection furnace with a capacity of 1 cubic meter, a normal time range of the suspension phase is 5 seconds to 5 minutes, most preferably approx. 10-30 seconds. If the suspension phase is too short it is difficult to lower the heating medium gas temperature to a desired level. On the contrary if it is too long total operation time is undesirably prolonged, though workpieces may have a better finish.
Also, for adjusting the furnace temperature, it is effective to vary a time ratio of the blowing phase and suspension phase of the heating medium. For example, in case where the furnace temperature exceeds a preset value a time ratio of the blowing phase is reduced, while when the furnace temperature becomes less than the preset value the time ratio of the blowing phase is increased.
For such purpose, the heating medium gas circulation path switching mechanism controlling means comprises means for correction for executing feedback control of a ratio of the blowing phase and the suspension phase in accordance with a temperature inside the brazing chamber.
The control correction means is provided with a sensor for detecting the furnace temperature and a programmable controller for varying the time ratio of the blowing phase and suspension phase according to the furnace temperature detected by the sensor. A preferable range of the time ratio of the blowing phase and suspension phase is 30:70 to 70:30.
According to the invention the heating medium gas is intermittently supplied at short intervals to perform the pulse heating, however a short-time suspension phase does not instantly lower the brazing chamber temperature because of a large thermal capacity of the brazing chamber. Therefore, it is effective to provide the brazing chamber with means for introducing cooling medium gas, by which a cooling medium gas is introduced into the brazing chamber during the suspension phase, for lowering the brazing chamber temperature in a short time to increase the effect of the pulse heating.
A low-temperature gas such as cooled carbon dioxide gas, nitrogen gas, argon gas, dried air, etc. may be used as cooling medium gas, among which the low-temperature nitrogen gas obtained by evaporating liquid nitrogen gas is most preferable, because of its low dew point (lower than xe2x88x9250xc2x0 C. when the gas temperature is 30xc2x0 C.).
The cooling medium gas introducing means comprises one or a plurality of nozzles disposed on an inner wall of the brazing chamber, for introducing a low-temperature cooling medium gas of 20xc2x0 C. to 50xc2x0 C. evaporated in a storing container of the cooling medium gas such as liquid nitrogen. In some applications, means for blowing the heating medium gas may be utilized as the cooling medium gas introducing means also.
The cooling medium gas introducing means also comprises means for controlling cooling medium gas flow, so that the cooling medium gas is introduced into the brazing chamber only during the suspension phase of the heating medium gas. Control of timing for introducing the cooling medium gas into the brazing chamber and of a flow of the cooling medium gas is executed by an electromagnetic valve connected with a programmable controller.
Also, for promoting a temperature drop in the brazing chamber during the suspension phase, the brazing chamber further comprises means for discharging exhaust heating gas, to be activated during the suspension phase of the blow of the heating medium gas.
The exhaust heating gas discharging means comprises an aspirator provided with a venturi tube to be operated by compressed air located on the outlet side of a discharging tube having an opening in the brazing chamber. The aspirator, which can lower the brazing chamber temperature in a short time by rapidly discharging hot air out of the brazing chamber during the suspension phase, can be operated at a substantially low running cost.
In the thermal medium gas heating chamber, it is preferable to provide a tube heater having a heat source of a fluid fuel, as means for heating the thermal medium gas composed of an inactive gas such as nitrogen etc. Usually the tube heater is heated by a gas burner with which a fuel gas such as propane is used, while naturally a liquid fuel burner can also be employed.
It is also preferable to employ a hydrogen gas burner in which hydrogen and oxygen are used, for heating the tube heater. The hydrogen gas and oxygen gas can be easily supplied to the hydrogen gas burner by installing a water electrolyzation device. Further, since a hydrogen gas burner does not generate a combustion exhaust gas such as carbon dioxide, sulfur oxide, nitrogen oxide or dust etc., using a hydrogen gas burner is also preferable from an environmental viewpoint.
An electric heater may also be employed as a heat source for the tube heater. Referring to a type of the electric heater, it is preferable to use a sheath heater or a coil heater.
Further, the thermal medium gas heating chamber may comprise a high-frequency induction heating device having a multitude of radiating projections, as means for heating the heating medium gas. The high-frequency induction heating device employed for such purpose is based on a similar principle and of a substantially similar constitution to that of an air-cooling heat sink for cooling a semiconductor device etc.
The heat sink transfers heat of a heating element of a semiconductor device etc. through the air for cooling the semiconductor device etc., while the high-frequency induction heating device according to the invention heats a metal heating plate with an electromagnetic coil, and transfers the heat to the thermal medium gas through the metal radiating projections, thus to heat the thermal medium gas.
The high-frequency induction heating device is more advantageous from the viewpoint of prevention of pollution than the tube heater since it does not generate exhaust gas, and enables easy and accurate control of the heating medium gas temperature simply by on/off operation of a high-frequency power source, therefore it is extremely advantageous to employ the high-frequency induction heating device as heat source in the uniform heating process according to the invention.
The heating plate of the high-frequency induction heating device may be constituted with a metal plate having a high thermal conductivity such as copper or aluminum etc. and a multitude of radiating projections of the same material attached the metal plate. The electromagnetic coil may be a metal coil of copper etc. connected with the high-frequency power source. Constituting the metal coil in a form of a hollow pipe and supplying cooling water through such pipe can generate a high power of 1 to 5 kw.
Shape of the radiating projections may be arbitrarily determined including bar-shape, plate-shape etc., while it is preferable to employ corrugated fins. Also, it is preferable to arrange the projections for example in a hound""s tooth pattern, so that a longer contact time can be secured with the thermal medium gas.
The thermal medium gas heating chamber is provided with an ordinary blower as means for blowing heating medium gas. A rotary wing fan of either forced draft type or suction draft type may be employed as a blower, among which a forced draft fan such as a turbo fan is preferably employed.
The heating medium gas blowing means is attached to an inner wall of the brazing chamber and comprises a multitude of nozzles directed toward the workpieces. Number of nozzles may vary depending on the dimensions of the brazing chamber, while it is preferable to provide 30 to 200 pieces on one side, i.e. 60 to 400 pieces on both sides for a dual-blowing type brazing chamber. Also it is preferable to arrange the nozzles regularly, for example in a check, hound""s tooth or concentric pattern when the wall is of a square shape.
Each nozzle of said heating medium gas blowing means is constructed so that the blowing direction of said heating medium gas can be adjusted freely. With concrete, the neck swing nozzle can be used in which it is possible to turn the neck portion 380 degrees.
When the blowing directions of all or a part of the nozzles being on the inner wall surface of the brazing chamber are turned to the specific area of workpieces, it can be heated concentrated the specific area of workpieces. When the blowing directions of all or a part of nozzles of both sides of the brazing chamber are deflected for 10-20 degrees against the inner wall surface, a rotational flow of the heating medium gas is produced in the brazing chamber, as the result, the uniform heating of workpieces is realized.
The heating medium gas blowing means may be optionally disposed on each of the brazing chamber walls, in other words 1 to 6 units may be provided in case of a cubic brazing chamber. Still, it is recommendable to dispose two units of heating medium gas blowing means on opposing vertical walls of the brazing chamber, so that the heating medium gas is blown toward the workpieces from both left and right directions. Further, it is also preferable to add another heating medium gas blowing means that blows the heating medium gas from upper the workpieces, thus to constitute a tri-directional blow.
The heating medium gas blowing means may be disposed in more than four directions in case of a batch type furnace.
The nozzles of the heating medium gas blowing means are projecting into the furnace and have an opening of approx. 8 mm, and it has been experimentally proven that under such configuration the heating medium gas can be injected through these nozzles at a speed of 12 to 14 m/s in the ordinary temperature, and speed variation of the heating medium gas injected from the 30 to 200 pieces of nozzles per wall is minimal, which leads to a conclusion that such heating medium gas blowing method is remarkably effective for uniform heating of each side of the workpieces.